Iron is an essential nutrient for the growth of all organisms and has very low solubility under physiological conditions. The ability to acquire iron under low-iron conditions is related to the virulence of bacterial pathogens. Mechanisms for acquisition of iron by the pneumococcus under low-iron conditions were investigated. Siderophore production was not detected by either chemical or biological methods. Its utilization of iron-containing compounds found in human hosts, e.g. transferin, hemoglobin, was tested. Both hemin and hemoglobin supported the full growth of pneumococci in a culture lacking another iron source, while human lactoferrin and transferrin failed to do so. To evaluate the significance of hemin utilization in the pathogenesis of pneumococcal infection, a mutant ST330 defective in hemin utilization was isolated by chemical mutagenesis. The virulence of the hemin utilization mutant in mice was analyzed. Mice were challenged with cells of pneumococcal type 2 and mutant ST330